Acidic Sulfate Soils

Abstract

Acidic sulfate soils are formed from potential acidic sulfate soils that are characterized by the accumulation of pyrite (FeS₂). Upon drainage of these soils, the pyrite oxidizes to produce sulfuric acid and hence acidic sulfate soils. The acid formed commonly decreases the soil’s pH to less than 4, and sometimes the pH becomes as acid as 2. The acid produced has a major effect on chemical and microbial processes in soils. Substances detrimental to plant growth, such as Al³⁺, Fe²⁺, H₂S, and CO₂ are often generated in amounts toxic to plant growth.

In acidic sulfate soils, the potential for very acidic soil conditions to develop results from natural processes. Often, however, cultural disturbances of these potential acidic sulfate soils, such as drainage for agricultural purposes or other reasons, cause the chemical and microbial processes to occur that result in development of severe acidic conditions. Acidic sulfate soils occur worldwide in most climatic zones but are found primarily in tropical regions. Most acidic sulfate soils are found in coastal areas at low elevations, where they developed from pyrite-containing marine sediments. These soils are a major problem because of their harmful effect on crop production, and because of the large areas affected in regions of the world where additional food production is especially important. Acidic sulfate soils are generally far more acidic than can be generated by even severe acidic precipitation problems. Acidic precipitation is unlikely to affect adversely either acidic sulfate soils or, for different reasons, potential acidic sulfate soils.

Knowledge of acidic sulfate soil chemistry, the problems experienced with such soils, and feasible management practices to mitigate some of the problems should provide information on the type and direction of chemical changes that may occur in poorly buffered soils in other regions of the world as a result of acidic precipitation.